Add handlePaste tests for testnet/regtest and identity URIs

Fills in the handlePaste cases from #534 (closed) that were still untested. The existing AppTest block covers empty input, mainnet addresses, amounts, bad amounts, unknown scheme and the http:// wrapper; this adds:

• nexatest: and nexareg: address detection (chain comes from the URI prefix, and the nexareg: case also carries an amount)
• the https:// wrapper in both unwrap forms: nexa/<address> → nexa:<address>, and the non-blockchain nexid/<host> → nexid://<host>
• the nexid: identity branch: op=login against an unregistered domain (errors, stays put) and a registered one (goes to IdentityOp), op=reg for a new domain (goes to Identity with newDomain set and the requested perms parsed), op=sign (any account is a candidate, no associated accounts), and a URI with no op at all

The identity tests assert on where nav lands and on the resulting IdentitySession, since handlePaste returns true for any nexid: URI regardless of what HandleIdentity decides.

Verified locally: :shared:jvmTest --tests "ui.AppTest" — 43 passed, 0 failed.

Closes #534 (closed)